In wireless communication systems, such as defined by 3GPP Long Term Evolution (LTE/LTE-A) specification, user equipments (UE) and base stations (eNodeB) communicate with each other by sending and receiving data carried in radio signals according to a predefined radio frame format. Typically, the radio frame format contains a sequence of radio frames, each radio frame having the same frame length with the same number of subframes. The subframes are configures to perform uplink (UL) transmission or downlink (DL) reception in different Duplexing methods. Time-division duplex (TDD) is the application of time-division multiplexing to separate transmitting and receiving radio signals. TDD has a strong advantage in the case where there is asymmetry of the uplink and downlink data rates. Seven different TDD configurations are provided in LTE/LTE-A systems to support different DL/UL traffic ratios for different frequency bands.
FIG. 1 (Prior Art) illustrates the TDD mode UL-DL configurations in an LTE/LTE-A system. Table 100 shows that each radio frame contains ten subframes, D indicates a DL subframe, U indicates an UL subframe, and S indicates a Special subframe/Switch point (SP). Each SP contains a DwPTS (Downlink pilot time slot), a GP (Guard Period), and an UpPTS (Uplink pilot time slot). DwPTS is used for normal downlink transmission and UpPTS is used for uplink channel sounding and random access. DwPTS and UpPTS are separated by GP, which is used for switching from DL to UL transmission. The length of GP needs to be large enough to allow the UE to switch to the timing advanced uplink transmission. These allocations can provide 40% to 90% DL subframes. Current UL-DL configuration is broadcasted in the system information block, i.e. SIB1. The semi-static allocation via SIB1, however, may or may not match the instantaneous traffic situation. Currently, the mechanism for adapting UL-DL allocation is based on the system information change procedure.
In 3GPP LTE Rel-12 and after, the trend of the system design shows the requirements on more flexible configuration in the network system. Based on the system load, traffic type, traffic pattern and so on, the system can dynamically adjust its parameters to further utilize the radio resource and to save the energy. One example is the support of dynamic TDD configuration, where the TDD configuration in the system may dynamically change according to the DL-UL traffic ratio. When the change better matches the instantaneous traffic situation, the system throughput will be enhanced. For example, in one scenario, multiple indoor Femto cells deployed on the same carrier frequency and multiple Macro cells deployed on an adjacent carrier frequency where all Macro cells have the same UL-DL configuration and the indoor Femto cells can adjust UL-DL configuration. In another scenario, multiple outdoor Pico cells deployed on the same carrier frequency and multiple Macro cells deployed on an adjacent carrier frequency where all Macro cells have the same UL-DL configuration and the outdoor Pico cells can adjust UL-DL configuration.
FIG. 2 (Prior Art) illustrates an LTE/LTE-A mobile communication system 200 with adaptive TDD configuration. Mobile communication system 200 comprises a Macro base station eNB 201 serving Macro cell 1, base station eNB 202 serving small cell 2, and base station eNB 203 serving small cell 3. Cell 1 is a Macro cell and its TDD configuration is more static. Small Cells 2-3 are within the macro cell's coverage. Cell 2 and Cell 3 form an isolated cell cluster 1, where TDD configuration can be independently adjusted. All cells in an isolated cell cluster should apply the TDD configuration change together. In this example, assume cell 1 applies TDD configuration 5, which is configured semi-statically, and the isolated cell cluster, i.e. cell 2 and cell 3, originally applies TDD configuration 5. As more UL traffic is demanded in the isolated cluster, it changes the TDD configuration to TDD configuration 3.
The notification of TDD change in an adaptive TDD system may be sent through a dedicated signaling, i.e., RRC (Radio Resource Control), MAC (Media Access Control), or PDCCH (Physical Downlink Control Channel) signaling. One reason to adopt TDD configuration change by dedicated signaling is that it can be adjusted more efficiently and frequently to match the instantaneous traffic pattern. In an adaptive TDD system, however, there may be legacy UEs and new released UEs. If the TDD change is sent through the dedicated signaling, then only new released UEs understand the information. The legacy UEs cannot know the dynamic TDD configuration because they cannot interpret the new information element. As a result, the legacy UEs may interfere with the operation of other UEs. For example, a legacy UE3 may perform random access in its cognitive UL subframe, but the subframe is operated for DL transmission due to the TDD configuration change.
A solution is sought.